Insulation of ferromagnetic particles



Patented May 20, 1952 UNITED STATES PATENT OFFICE INSULATION F FERROMAGNETIC PARTICLES I George 0. Altmann, Elizabeth, N. J assignor to General Aniline & Film Corporation, New York,

N. Y., a corporation of Delaware No Drawing. Application June 1, 1949, Serial No. 96,619

9 Claims. (01. 1486.15)

This invention relates to a method for in- .sulating the particles of ferromagnetic metal powders, especially those obtained by thermal decomposition of metal carbonyls, particularly iron carbonyl, and to the products obtained thereby as well as molded articles, particularly magnetic cores, made from the resulting insulated powders and adapted for use in high frequency coils.

High-frequency magnetic cores are customarily made of ferromagnetic powders, generally iron or an iron alloy, characterized by high electrical conductivity. The high conductivity of the metal is the reason for using the metals in subdivided form and for employing particles of extremely fine subdivision in making such cores, since an increase in the particle size of the metal powder used in the core, or use of the metal in bulk, increases eddy currents to a point where they consume practically all of the power carried by a conductor forming a coil and providing the magnetic field of the core, thus nullifying the advantage of the core material. Increased subdivision of the core metal reduces the eddy current power loss substantially in proportion to the square of the average linear particle diameter. However, increased subdivision alone does not suflice. It is also necessary to surround a majority, if not practically all, of the particles with an electrical insulating layer.

Insulating layers for the particles of ferromagnetic powders can only be made of nonferromagnetic material. Accordingly, to avoid excessive reduction in the permeability of the core, it is desirable, and practically essential, to provide an insulating layer on the particles of minimum thickness, preferably equal to the thickness of only a few molecules. Unfortunately, very few known materials afford eilicient electrical insulation when applied in such thin layers. It is further required that the insulating material yield a fairly uniform layer on the particles; further, it should not attack the particles chemically and should have high physical strength and toughness so that it will not wear or rub off under strong compression; and finally, the insulation should resist changes under prolonged exposure to atmospheric conditions such as moisture and temperature variation.

A number of insulating materials, having some of the foregoing properties, and methods for applying them, have been suggested heretofore for ferromagnetic particles, but thus far, completely satisfactory insulation and a method therefor'has not been found. Thus, when applied to carbonyl iron powder of which the par- Z ticles are relatively small as compared with metal powders produced by other processes, even the best of the insulating materials and methods heretofore known fail to yield results affording the full advantages of the ultra-fine subdivision and superior magnetic characteristics of such powders. When aqueous solutions, containing, for example, a silicate or a phosphate, were applied to such powders, the iron tended to dis solve superficially and formed hydroxides or partly hydrated oxides on drying. Such hydroxides and hydrated oxides have a tendency to produce excessive losses in cores made from.

the powders, particularly at frequencies of 10 to 10" cycles per second. The presence of the oxides and hydroxides further manifests itself by serious reduction in the specific resistivity, the values obtained being only occasionally as high as 10 ohmcm., but usually less than 10 ohm-cm, and often 10 ohm-cm. or less. Particle insulation by use of phosphate solutions has a further disadvantage in that the results are difficult to control. Atmospheric moisture and mixing equipment materials often participate in the reaction, causing fluctuation in the results. Such variations constitute a serious difficulty when it is desired to manufacture large numbers of cores commercially in which it is required that such characteristics as the Q-value and permeability should remain practically uniform throughout prolonged periods of production to fulfill standards imposed by sharply tuned highfrequency circuits. It has proven extremely diflicult in the past to achieve the required uniformity.

It is an object of this invention to provide a relatively simple and economical method of insulating the particles of ferromagnetic metal powders, which avoids the foregoing disadvantages, decreases losses and increases resistivity without sacrificing any of the desired characteristics of the materials obtained, and which provides a highly uniform product.

It is a further object of the invention to provide insulated magnetic particles and molded articles such as cores produced therefrom which satisfy the exacting demands of uniformity, electromagnetic eiliciency and durability required in commercial production.

In accordance with this invention, the particles of a ferromagnetic metal powder are first coated by mixing the powder with an aqueous solution containing a water-soluble silicate and a water-soluble or water-dispersible carbohydrate. After drying in such a manner as to form a coating on the individual particles of the silicate and carbohydrate, the metal powder is moistened with a volatile, water-miscible, organic liquid and a small amount of aqueous phosphoric acid is added to the mixture. After thorough mixing, the powder is dried.

Water-soluble silicates employed in the initial step of the aforesaid treatment are, for example, alkali metal silicates, such as sodium or potassium silicate, and suitable carbohydrates include, for example, sucrose, dextrose, levulose, lactose, maltose, as well as starch or cellulose in the form of colloidal aqueous solutions. In the second treatment step, water-miscible, readily volatile organic solvents suitable for use include, for example, lower alcohols such as methanol, ethanol, propanol, or isopropanol, ketones such as acetone or ethyl methyl ketone, and the like.

The phosphoric acid is employed preferably in concentrated aqueous form, (e. g. at least 50% H3PO4), and can be used alone or may contain minor amounts of other acids or salts which do not attack the ferromagnetic particles and do not interfere with the formation of phosphates on the surfaces thereof.

While it is not desired to limit this invention to any theory, it is probable that oxides and hydroxides formed on the metal during the silicatecarbohydrate treatment are converted by the phosphoric acid treatment to phosphates,

thus eliminating the high power losses and low 7 resistivity characterizing powders bearing the oxides or hydroxides. The silicate-carbohydrate treatment apparently insures formation of a thin coating which results in relatively high permeability. Atmospheric moisture and other influences which cause deterioration of phosphate coatings have practically no eifect on the insulation provided by the process of this invention. Resistivities obtained with carbonyl iron powder treated in accordance with this invention are consistently above 10 ohm-cm.

The following examples illustrate the preparation of insulated carbonyl iron powders and cores prepared therefrom in accordance with this invention. Parts are by weight unless otherwise indicated, and parts by volume signify the volume of an equal number of parts by weight, of water (e. g. if 1 gram is selected as the unit part by weight 50 parts of iron powder" signifies 50 grams of iron powder, and in the same procedure, a quantity expressed in terms of parts by volume, as, for example, parts by volume of aqueous solution or 15 parts by volume of acetone signifies a volume ofthese liquids equal to that of the same number of parts by weight of water, i. e., 15 ml).

EXAMPLE 1 50 parts of iron powder obtained from thermal decomposition of iron pentacarbonyl, of which the particles are spherical and have a weightaverage diameter of about 3 microns, were placed in a shallow dish and mixed with 15 parts by volume of an aqueous solution of 0.25 part of sodium silicate and 0.05 part of cane sugar. The iron powder was thoroughly mixed with the solution by mechanical agitation and heated during the mixing by means of an infrared lamp to evaporate water. When the mixture appeared completely dried, it was baked at 170 C. for 1 hour to remove residual moisture. After cooling, the powder was moistened with 15 parts by volume of acetone. The resulting slurry was agitated and 0.1 part by volume of 85% aqueous phosphoric acid was slowly added. The mixture was heated by exposure to infrared radiation with continuous agitation to evaporate the acetone and residual moisture, yielding a p wder of which the particles were individually insulated, suitable for molding into cores and other magnetic objects.

For purposes of comparison, an additional portion of the same carbonyl iron powder was insulated by treatment with a sodium silicate-cane sugar solution as described above, except that the amount of sucrose was increased to 5 times the amount above specified to insure adequate coverage. The powder was dried by evaporating the water to yield an insulated powder, but treatment with phosphoric acid was omitted. A second control portion of carbonyl iron powder of the type described above was treated with acetone and phosphoric acid and then dried as described above, without preliminary coating with silicate and cane sugar.

Magnetic cores were prepared by mixing the powder obtained in accordance with the proce dure of this example, as well as from the two control samples, by mixing with a binder in a volatile solvent therefor (e. g. an acetone solution of a furfural-formaldehyde condensation product such as Durite 275") employin for example, about 1 part by volume of a 25% solution of the binder to 5 parts of iron powder. After evaporating the solvent, the powders were mixed with a small amount (e. g. 0.5%) of a lubricant (particularly a high melting wax such as fA'crawax C), and the powders were then molded into cores by subjecting them to compression in a die, at about 20 tons per square inch, yielding products of equal densities which werebaked, e. g. at about 170 C. for one hour. to cure the binder. I

The resulting cores were tested in high-irequency coils at various frequencies covering a large portion of the electromagnetic spectrum. The Q-value and tuning capacitance were obtained directly from a Q-meter, and the resistivity was calculated from the resistance measured between the ends of the core held in contact with mercury. The results obtained are set out in the following table, wherein the data for the core produced from powder insulated in accordance with this invention as described in this example is given in column A, while the data for the cores produced from the two control samples, respectively, are given in columns B and C.

Carbonyl iron powder of the same type described in Example 1 was insulated in the same manner, except that the proportions of sucrose and of o-phosphoric acid were increased, respectively, to 0.15 part and to 0.3 part by volume, and the insulated powder was made into a core as described above and tested in like man ner. Results were as shown in the following table:

Re istivity (in ohm-cm.) X10 In repeating the preparation of the insulated owders in accordance with this invention as described in the foregoing examples, highly uniform characteristics were obtained, making possible the production of cores having uniform characteristics for high-frequency applications.

Insulated electromagnetic powders in accord-- ance with this invention are preferably prepared from carbonyl iron powder, in which the particles are generally spherical and usually have a weightaverage diameter from 3-20 microns. However, the method of insulation of this invention can also be applied with similar advantages to powdered ferromagnetic alloys prepared, for example, by thermal decomposition of mixtures of the corresponding carbonyls with iron carbonyl; or to powdered iron or ferromagnetic alloys prepared by other methods such as reduction with gases (especially with hydrogen) of powdered oxides (especially iron oxide), or by electrolytic deposition, the particle size of suitable powders obtained by these other methods often averaging somewhat more than the aforesaid carbonyl iron range, e. g. up to or microns.

In the initial treatment of the invention, other water-soluble silicates, particularly alkali metal silicates, can be used instead of sodium silicate (e. g. potassium silicate) and other carbohydrates (e. g. dextrose, levulose, maltose, lactose), or polymeric carbohydrates in colloidal aqueous solution (e. g. starch, glycogen, inulin, dextrin, or cellulose) can be used.

For the aforesaid ferromagnetic metal powders, and especially carbonyl iron powders in which the particles have an average diameter of 320 microns, at least 0.1 part of water-soluble silicate and at least 0.05 part of carbohydrate are employed for each 100 parts of metal powder (i. e., the silicate amounting to at least 0.1%, and the carbohydrate, to at least 0.05% of the weight of the metal powder). In the second treatment step, the amount of phosphoric acid (H3PO4) employed should be at least 0.05 part per 100 parts of metal powder (i. e., 0.05% by weight of the metal powder). Amounts of the treating materials in excess of those illustrated in the examples can be used, the upper limits being of a practical rather than a critical nature. Thus, the amounts of alkali metal silicate and of watersoluble carbohydrate can be raised to as much as ten times the minimum proportions specified above (i. e., alkali metal silicate amounting to 1.0%, and carbohydrate amounting to 0.5% of the weight of the metal powder) without impairing the properties of the insulated powder, and the amount of phosphoric acid (H3PO4) can be increased to six times the minimum amount above specified (i. e., 0.3% of the weight of the metal powder).

The amount of water employed in the first treatment step and of volatile water-miscible solvent employed in the second step should be sufficient to insure thorough permeation of the powdered mass and hence, uniform distribution of the insulating materials on the particle surfaces. Thus, 10 to 30 parts by volume of water can be used in the first step and a similar volume range of volatile solvent such as acetone can be used in the second step in the aforesaid treatment of each parts of metal powder, the proportion of water or solvent thus being 10 to 30 milliliters per 100 grams of metal powder.

Drying or evaporation of the water and of the volatile solvent can be carried out by heating in various ways in addition to the use of infrared radiation illustrated in the examples. Thus, the evaporation may be carried out by subjecting the moistened powder to agitation in a current of heated gas, or by agitating in vacuo while heating externally. To complete the drying, the powder can be baked at temperatures, for example, of to 200 C. It is often advantageous to carry out the drying in the absence of air or other oxygen-containing gases.

In preparing cores from the insulated powder produced in accordance with this invention, other binders can be used instead of the furfural-formaldehyde condensation product of the examples. For example, urea-formaldehyde or phenol-formaldehyde condensation products can be used, or varnishes, drying oils, vinyl polymers and the like, in admixture with suitable volatile organic solvents therefor, which effect distribution of the binder throughout the mass of insulated particles. The proportion of the binder is preferably of the order illustrated in the example, i. e., about 5 parts of binder per 100 parts by weight of powder.

The mixture of powder and binder is molded in a die under pressure to form cores, pressures of 5 to 100 tons per square inch being satisfactory. When heat-curable binders such as furfuralformaldehyde, urea-formaldehyde or phenolformaldehyde binders are used, the molding is preferably carried out without heating, and the binder is subsequently cured by baking at an appropriate temperature, such as C. or from 150 to 200 C. When thermoplastic binders are used, the molding may be carried out at elevated temperatures suitable for softening the binder into moldable form.

The molded articles obtained, particularly cores for high-frequency coils, are characterized by uniformity not heretofore obtained, superior electromagnetic characteristics such as an unusually high Q-value and high resistivity, with good permeability and low eddy current losses at frequencies ranging as high as 10 cycles per second and above.

Variations and modifications which will be obvious to those skilled in the art can be made in the procedures hereinbefore specifically described without departing from the scope or nature of the invention.

I claim:

1. A process for insulating the particles of a ferro-magnetic metal powder, which comprises coating the particles with an aqueous solution of a water-soluble silicate and a water-soluble carbohydrate, the amount of silicate being at least 0.1%, and of carbohydrate at least 0.05 of the weight of the metal powder, and the amount of water at least 10 milliliters per 100 grams of said powder; and drying by heating to form a thin coating of the carbohydrate and the silicate on the individual particles; thoroughly mixing the resulting powder with a volatile water-miscible organic solvent amounting to at least 10 milliliters per 100 grams of the powder, and a relatively small amount of concentrated aqueous phosphoric acid, containing H3P04 amounting to at least 0.05% of the weight of the powder, and evaporating the solvent and residual moisture from the powder.

2. A process for insulating the particles of a ferromagnetic metal powder, which comprises coating the particles with an aqueous solution of a water-soluble silicate and a water-soluble carbohydrate, the amount of said silicate being from 0.1 to 1%, and of said carbohydrate, from 0.05 to 0.5%, of the weight of the iron, and the amount of water, at least milliliters per 100 grams of the metal powder, and drying by heating to form a thin coating of the carbohydrate and silicate on the individual particles; thoroughly mixing the resulting powder with a volatile water-miscible organic solvent in suincient amount, at least 10 milliliters per 100 grams of said powder, to permeate the powder, and with concentrated aqueous phosphoric acid containing an amount of H3PO4 corresponding to 0.05 to 0.3% of the weight of the iron, and evaporating the solvent and residual moisture from the powder.

3. A process as defined in claim 2, wherein the powder bearing thecarbohydrate-silicate coating is baked at 150-200 C. for removal of residual moisture, before mixing with the organic solvent and phosphoric acid.

4. A process as defined in claim 3, wherein baking is eiiected in the absence or oxygen-containing gases.

5. A process for insulating the particles of carbonyl iron powder having a weight-average diameter of 3-20 microns, which comprises coating the particles with an aqueous solution containing a sugar in an amount from 0.05 to 0.5% and an alkali metal silicate in an amount from 0.1 to 1% of the weight'of the iron powder, and water amounting to at least 10 milliliters per 100 grams of the powder; drying the powder to form a thin coating of the sugar and silicate on the individual particles and baking at ISO-200 C. to remove moisture; thoroughly mixing the resulting powder with acetone in sufiicient amount, at least 10 milliliters per 100 grams of the iron powder, to permeate the powder, and with concentrated aqueous phosphoric acid containing an amount of HxPOa corresponding to 0.05 to 0.3% of the weight of the iron, and evaporating the solvent and residual moisture from the powder.

6. A process for insulating the particles 0! carbonyl iron powder having a weight-average diameter of 3-20 microns, which comprises coating the particles with an aqueous solution containing cane sugar in an amount from 0.05 to 0.5% and sodium silicate in an amount from 0.1 to 1% of the weight'of the iron powder, and water a ount- 8. Carbonyl iron powder of which the particles have an insulating coating formed by the process of claim 5.

9. A molded core consisting of carbonyl iron powder as defined in claim 8, and a heat cured thermosetting non conductive binder in an amount of the order of 5 parts per parts by weight of the powder, uniformly distributed among the powder particles and providing cohesion for said particles.

GEORGE O. ALTMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,789,477 Roseby Jan. 20, 1931 1,866,123 Neighbors July 5, 1932 1,873,599 Kappeler Aug. 23, 1932 1,982,690 Polydoroff Dec. 4, 1934 2,064,771 Vogt Dec. 15, 1936 2,306,198 Verweij Dec. 22, 1942 

1. A PROCESS FOR INSULATING THE PARTICLES OF A FERRO-MAGNETIC METAL POWDER, WHICH COMPRISES COATING THE PARTICLES WITH AN AQUEOUS SOLUTION OF A WATER-SOLUBLE SILICATE AND A WATER-SOLUBLE CARBOHYDRATE, THE AMOUNT OF SILICATE BEING AT LEAST 0.1%, AND OF CARBOHYDRATE AT LEAST 0.05%, OF THE WEIGHT OF THE METAL POWDER, AND THE AMOUNT OF WATER AT LEAST 10 MILLILITERS PER 100 GRAMS OF SAID POWDER; AND DRYING BY HEATING TO FORM A THIN COATING OF THE CARBOHYDRATE AND THE SILICATE ON THE INDIVIDUAL PARTICLES; THOROUGHLY MIXING THE RESULTING POWDER WITH A VOLATILE WATER-MISCIBLE ORGANIC SOLVENT AMOUNTING TO AT LEAST 10 MILLILITERS PER 100 GRAMS OF THE POWDER, AND A RELATIVELY SMALL AMOUNT OF CONCENTRATED AQUEOUS PHOSPHORIC ACID, CONTAINING H3PO4 AMOUNTING TO AT LEAST 0.05% OF THE WEIGHT OF THE POWDER, AND EVAPORATING THE SOLVENT AND RESIDUAL MOISTURE FROM THE POWDER. 